A transmission line is a system of conductors suitable for conducting electrical power or signals between two or more terminals. For example, commercial-frequency electric power transmission lines connect electric generating plants, substations and their loads. Telephone transmission lines interconnect telephone subscribers and telephone exchanges. Radio-frequency transmission lines transmit high-frequency electrical signals between antennas and transmitters or receivers.
The term "transmission line" is applied to conductors only when both voltage and current at one line terminus may differ appreciably from voltage and current at another terminus.
Depending on the configuration and number of conductors and the electric and magnetic fields about the conductors, transmission lines may be described as open-wire transmission lines, coaxial transmission lines, or strip transmission lines.
Open-wire transmission lines are used mainly for communication and power distribution, and are not germane to the present invention.
A coaxial transmission line 10 is shown in FIG. 2, and comprises a conducting cylindrical shell 12, typically a solid or braided wire conductor, surrounding an electrically-isolated, concentric center conductor 14, which is typically solid but may also be stranded or helically wound on a plastic or ferrite core. The center conductor 14 is typically supported by a solid polyethylene or polystyrene dielectric 16, or, in the case of air- or gas-dielectric transmission lines, the center conductor may be supported by ceramic or plastic beads or washers.
The purpose of coaxial construction is to have the shell prevent radiation losses and interference from external sources. The electric and magnetic fields are nominally confined to the space inside the outer conductor. Coaxial transmission lines are widely used in radio, radar, television, and similar applications.
A strip transmission line consists of a conductor above or below extended conducting surfaces. When the conductor is above a single ground plane, the transmission line is called an unshielded strip transmission line. When the conductor is between two ground planes, the transmission line is called a shielded strip transmission line.
When electric power is applied at a terminus of a transmission line, electromagnetic waves are launched and guided along the line. When the electric and magnetic field vectors are perpendicular to one another and transverse to the direction of the transmission line, this condition is called the principal mode or the transverse electromagnetic (TEM) mode.
Coaxial and strip transmission lines have certain drawbacks associated with them. Coaxial transmission lines are usually employed for short distance runs where the advantage of their flexibility outweigh their relatively high attenuation. Generally, coaxial cable is not used above 3 GHz because of reduced power capability and the small dimensions necessary to avoid higher order propagation modes other than TEM mode. Strip transmission lines are usable at higher frequencies than coaxial transmission lines, but their frequency range depends on the width of the strip and the placement of mode suppression screws to suppress transmission modes other than TEM mode.
It is an object of the present invention to provide a transmission line that avoids the drawbacks of prior transmission lines and allows the efficient construction of a small size, high performance transmission line featuring low RF loss, low dispersion, and high reproducibility. The present invention can be used in EW (electronic warfare) radar, and other systems requiring low dispersion, high phase-accuracy delay lines with wide bandwidth and wide dynamic range.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.